1. Field of the Invention
This invention relates to a method for utilizing well log data and other measurements to determine the distance and direction between a cased blown out well and an adjacent open (e.g., uncased well) relief well. The intersection of a blown out well by a relief well is frequently required in order to kill the wild well by pumping cement or other fluids into its well bore. It is an extremely difficult and complex engineering problem to effect an intersection in the subsurface at depths frequently greater than 15,000 feet. This problem is much simpler if a method is available for determining the distance of the relief well to the casing during the course of its approach to the target well. The invention is also concerned with the use of similar data and measurements to avoid having two wells intersect, which can be a problem where the wells are drilled from a common starting point such as an off-shore drilling platform.
2. Description of the Prior Art
A number of methods have been proposed for utilizing well logging data to estimate distance and direction from an open well to a cased well. Direction and distance to casing can be estimated by lowering a magnetometer on a wireline into the relief well and measuring the magnetic field and/or its gradient produced by the natural magnetization of the casing. This method is described in U.S. Pat. No. 4,072,200.
A similar method is described by J. D. Robinson and J. P. Vogiatziz in U.S. Pat. No. 3,725,777 and in a publication entitled "Magnetostatic Methods For Estimating Distance And Direction From A Relief Well To A Cased Borehole", Journal of Petroleum Technology, v. 24, no. 6, pp. 741-750, June 1972.
A limitation of these methods is that the magnetic field and/or its gradient can be too weak for accurate detection unless the magnetometer is near the end of the casing where the field is strongest. This method is a passive method since it depends on the casing having a natural magnetization. Professor Arthur F. Kuckes of the School of Applied and Engineering Physics at Cornell University has recently developed two active methods for estimating distance and direction from a relief well to a cased target well.
The first method developed by Professor Kuckes and co-workers involves the creation of a low frequency alternating current in the casing of the blown out well by placing electrodes on the surface in the ground near the wild well. The conductivity of the metal casing is roughly 10.sup.6 times greater than that of the earth so that the current finds it favorable to flow in the casing. Thus, having established an alternating current in the casing the magnetic field produced by this current is measured by a magnetometer placed in the relief well on a wireline cable. Suitable interpretation of the direction and magnitude of the measured magnetic field can be used to estimate both distance and direction to the cased target well. A practical limitation of this method occurs because a blow out frequently results in the separation of casing joints. A result of this is that current leakage from the casing can prevent the induced currents from reaching desired depths with sufficient strength to produce measurable and interpretable magnetic fields.
Professor Kuckes and co-workers have also developed a second active method. This method places both the source current electrodes and the magnetometer in the relief well on a wireline logging cable. In this case the magnetometer measures a reflected magnetic field due to the presence of the casing. This method has serious interpretation problems if the relief well trajectory is not a straight line path. In practice, it is more common for a well to have a tortuous trajectory in space over distances of hundred to thousands of feet. In this case the electrodes producing the current and the magnetometer will not be the same straight line. This results in a primary and secondary magnetic field at the magnetometer. The primary field would be zero if the relief well trajectory were a straight line path.
In meandering wellbores the total magnetic field at the magnetometer consists of both the primary field and the secondary field. The secondary field results from the presence of the casing and therefore it is this field which contains information about the distance and direction of the relief well to the blown out well. In relief wells with tortuous paths it may not be possible to separate the primary and secondary fields with sufficient accuracy to obtain accurate distance and direction estimates using this method.
A different method which is more closely related to the method of this invention is disclosed in U.S. Pat. No. 3,748,574 and described in a publication by F. R. Mitchell et al entitled "Using Resistivity Measurements to Determine Distance Between Wells" printed in the Journal Of Petroleum Technology, v. 24, no. 6, pp. 723-741, June, 1972. This patent and publication represent the state of the prior art most relevant to the present invention. The method of the Mitchell et al publication utilizes logging data obtained in the relief well with an Ultra-Long Spaced Electric Log (ULSEL), electrical survey (ES) and induction and/or other shallow investigating resistivity logs. The latter logs are common and familiar to operators engaged in the drilling of oil and/or gas wells. The ULSEL tools were developed and are offered as a service of Schlumberger Well Surveying Corporation. These tools are electrode-type electrical logging tools which have electrode spacings of hundreds to thousands of feet. They were originally developed to provide a resistivity logging method which could have a depth of investigation of hundreds to thousands of feet.
A method for using the ULSEL data to detect salt domes at distances of hundreds to thousands of feet from a well has been described in U.S. Pat. No. 3,256,480. This patent contains a description of the ULSEL tools. The application of the ULSEL tools to salt dome detection was the original impetus for the development of the tool. Salt domes, as is well known, represent structural traps for hydrocarbons and therefore their presence, location and size are of obvious interest to geologists. The ULSEL can detect the presence of salt domes because these structures have anomalously high resistivities as compared to other geological structures present in the subsurface.
The aforementioned publication by Mitchell et al and U.S. Pat. No. 3,748,574 describe a method and field examples wherein ULSEL, ES and induction log data are utilized to estimate distance of a relief well to a cased target well. The ULSEL can be used to detect the presence of casing since in proximity to casing the resistivities measured by the ULSEL are reduced significantly relative to their values in the absence of casing. This reduction in measured resistivities when the casing is within the range of investigation of the ULSEL is a result of the fact that the resistivity of the casing is roughly 10.sup.-6 times that of the earth.
Mitchell et al discuss a method for estimating distance to casing by utilizing the ratio of measured to expected (e.g., theoretically computed in the absence of casing) resistivity. This ratio decreases as the casing is approached by the relief well. A method is proposed for using the aforementioned resistivity ratio to estimate distance to the cased target well. The method proposed by the aforementioned Mitchell et al publication and U.S. Pat. No. 3,748,574 can be described as follows. The relief well is logged with an induction tool in an interval of interest which in practice could range from tens to thousands of feet. Since the depth of investigation of induction logging tools is relatively shallow, the measured resistivities are not affected by proximity to casing provided that the relief well is greater than roughly twelve feet from the target well. The induction log resistivities are used to make a layered resistivity model of the earth which best approximates the actual resistivity versus depth profile determined from the induction log. The number of layers included in the model is variable and in practice could be several hundred.
The effects of resistivity anisotropy are included in the layered model by having, in each layer, a resistivity parallel to the layering planes and a resistivity perpendicular to the layering planes which are not equal in an anisotropic layered earth. Since only a finite number of layers can be included in the model, it is assumed that the layered medium is bounded above and below by semi-infinite half spaces with appropriately chosen resistivities. The theoretically expected response of the ULSEL is calculated for this layered medium. This then gives the theoretical response of the ULSEL in the absence of the cased target well. The method of Mitchell et al assumes that the relief well trajectory is a straight line and therefore that the direction of the relief well has a constant bearing (e.g., angle measured relative to north) and deviation (e.g., angle measured relative to the vertical). A limitation of this method is that in practice a well borehole frequently follows a meandering trajectory in space. Failure to take into account this trajectory leads to errors in the theoretically computed ULSEL responses and, therefore, to errors in distances estimated from the ratio of measured to expected resistivities.
Another limitation of this method is that directional information canot be obtained using the assumption of a straight line path for the relief well. The second step in the Mitchell et al method utilizes the ratio of measured to expected resistivities to estimate distance of the relief well to the cased target well. The step of the method makes an unrealistic and inconsistent approximation. The approximation is that the cased target well is not in a layered medium but is situated instead in an infinite homogeneous medium. This approximation is not valid, is inconsistent with the first step of the method which constructs the layered medel, and therefore it introduces unknown errors in the distances estimated using this method.